Herpes simplex virus 1 (HSV-1) is a nuclear-replicating, double-stranded (ds) DNA virus. HSV-1 genes are expressed in a coordinate manner. The tegument protein VP16 first activates expression of the five immediate-early (IE) genes. Two IE proteins, ICP4 and ICP0, then activate early (E) and late (L) gene expression. The mechanisms whereby VP16 activate IE gene expression are so well characterized that VP16 is used as a tool to study gene expression in general. In contrast, the mechanisms whereby ICP4 and ICP0 activate gene expression are not fully understood. ICP4 is the only essential HSV-1 transcription activator. HSV-1 genomes are not chromatinized in the virion, but are chromatinized in the nucleus. The basic unit of chromatin is the nucleosome, composed of two each of the core histone dimers H2A-H2B and H3-H4 wrapped by 146 base pairs of dsDNA. Chromatin is dynamic, as histones disassemble from nucleosomes, diffuse through the free pool bound by chaperones, and reassemble nucleosomes at different sites. HSV-1 chromatin is more dynamic than cellular chromatin. Cellular chromatin dynamics are altered by the incorporation of variant histones in place of canonical ones. Variant H3.3 is enriched in nucleosomes assembled with DNA of transcribed genes or telomeres, and nucleosomes containing H3.3 are more dynamic than those assembled with H3.1. Variants macroH2A and H2A.B are preferentially enriched in nucleosomes assembled with DNA of silenced or transcribed genes, respectively. MacroH2A assembles less dynamic nucleosomes than canonical H2A, whereas H2A.B assembles more dynamic ones. Upon nuclear entry of HSV-1 genomes, the total amount of nuclear DNA increases and histone synthesis is inhibited. As the number of histone binding sites increases but the amount of histones does not, we would expect the histone free pools to decrease in HSV-1-infected cells. However, it increased. Nuclear entry of HSV-1 genomes is required to enhance histone free pools, but HSV-1 DNA replication is not. We proposed a model in which a cellular defense mechanism chromatinizes HSV-1 genomes to silence HSV-1 gene expression. To counteract silencing, HSV-1 evolved proteins that prevent or disrupt the stable chromatinization of their genomes. My hypothesis is that these proteins are HSV-1 transcription activators. In this thesis, I show that HSV-1 VP16 and ICP0 mutants still enhanced histone dynamics, though less so than the wild type virus, whereas ICP4 mutants barely enhanced them. To test whether ICP4 enhanced histone dynamics directly, I evaluated histone dynamics in cells co-expressing fluorescently tagged histones and full length or truncated, transcriptionally inactive, ICP4. The dynamics of H2B and H4, which have no variants and thus represent the H2A-H2B and H3-H4 dimers, were enhanced in cells expressing detectable levels of full length, but not truncated, ICP4. The dynamics of H3.1 and H3.3, which assemble less or more dynamic nucleosomes, respectively, were both enhanced in ICP4-expressing cells. Whereas H3.1 had granular distribution in cells expressing undetectable levels of ICP4, consistent with the incorporation of H3.1 in chromatin, it was diffusely distributed in ICP4-expressing cells. H3.3 distribution was not affected by ICP4. The dynamics of H2A, macroH2A, H2A.X, or H2A.Z did not change in ICP4-expressing cells, but those of H2A.B were enhanced. The distribution of H2A.B was altered in ICP4-expressing cells, with greater H2A.B enrichment at the nucleolus, where ICP4 itself localized. Histones in the nucleolus are more dynamic than those in the cellular chromatin. The increased dynamics of H2A.B in ICP4-expressing cells may thus be a result of the greater amount of H2A.B in the most dynamic population. The nucleolus is disassembled in HSV-1-infected cells. Whereas H2A, macroH2A, and H2A.X are mostly depleted from the replication compartments, H2A.B was less so. The dynamics of all H2A variants except for H2A.B increased in HSV-1-infected cells. In cells infected with an HSV-1 mutant encoding truncated ICP4, n12, the replication compartments do not form and the nucleoli are fragmented. H2A.B is displaced from the fragmented nucleoli in n12-infected cells, but its dynamics do not decrease. Functional ICP4 is thus not required to displace H2A.B from the nucleolus, but is required to decrease H2A.B dynamics. In this thesis, I show that ICP4 is the major HSV-1 modulator of histone dynamics. I suggest a model in which ICP4 activates transcription by maintaining the HSV-1 genomes in a most dynamically chromatinized and transcriptionally competent state.

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